1. Field of the Invention
This invention relates to a non-linear frequency conversion apparatus, and particularly to a non-linear frequency conversion apparatus with two frequency conversion crystals, with only one crystal providing frequency conversion for a selected portion of a tuning range, while the other serves as a beam displacement compensator.
2. Description of the Related Art
Non-linear optical devices, such as harmonic generators and parametric devices, provide a method of extending the frequency range of available input beams from coherent sources such as lasers. Frequency conversion is a useful technique for extending the utility of high power lasers. It utilizes the non-linear optical response of an optical medium in intense radiation fields to generate new frequencies. It includes both elastic, optical-energy-conserving, processes, such as harmonic generation, and inelastic processes, such as stimulated Raman or Brillouin scattering.
There are several commonly used elastic processes. Frequency doubling, tripling, and quadrupling generate a single harmonic from a given fundamental high-power source. The closely related processes of sum and difference frequency generation also produce a single new wavelength, but require two high power sources. These processes have been used to generate high power radiation in all spectral regions, from the ultraviolet to the far infrared. Optical parametric devices and amplifiers generate two waves of lower frequency. They are capable of generating a range of wavelengths from a single frequency source, in some cases spanning the entire visible and near infrared regions.
High efficiency second harmonic conversion depends on parameters which are related to the input beam source, such as power density, beam divergence, and spectral line width, and parameters associated with the harmonic generator, such as the value of the non-linear coefficient, crystal length, angular and thermal deviation from the optimum operating point, absorption, and inhomogeneities in the crystal.
An effective method of providing equal phase velocities for the fundamental and second harmonic waves in the non-linear medium utilizes the fact that dispersion can be offset by using the natural birefringence of uniaxial or biaxial crystals. These crystals have two refractive indices for a given direction of propagation, corresponding to the two allowed orthogonally polarized modes. When the phase mismatch between the fundamental wave and the second harmonic wave for collinear beams is zero index phase matching occurs.
The parameters of the harmonic generator which affect doubling efficiency are temperature, phase matching angle, absorption, optical homogeneity, non-linear coefficient and figure of merit.
Current devices for frequency converting tunable sources, including but not limited to dye lasers, Ti:sapphire lasers and optical parametric devices, use a single non-linear crystal or a set of non-linear crystals. The number and type of crystals used depends on the non-linear materials' phase matching properties and the wavelength ranges being covered.
By way of specific example, to frequency double the signal (.about.400-700 nm) and/or the idler (700-800+ nm) of an optical parametric device in order to generate second harmonic output from .about.200 to .about.400 nm, most of the tuning range can be covered by using just two Type I cut BBO crystals, and each is rotated over a fairly modest, 10-15 degrees, angular range. The BBO crystal provides frequency conversion, and a second optic is utilized as a beam displacement compensator to cancel out beam translation effects. The crystal is rotated to maintain the proper phase matching angle while the compensator is counter rotated to cancel out beam translation effects. In many instances, some form of automatic tracking is incorporated to keep the crystal angle optimized as the system is tuned. If the angular range available from a single crystal is insufficient to cover the desired tuning range, then a new crystal with a different angular cut is substituted into the beam path to extend the second harmonic generation tuning range.
There are additional limitations to current frequency conversion devices. One limitation is that it has not been possible to obtain crystals with large enough apertures, to achieve a wide range of tuning.
The use of two crystals in a stacked optical parametric oscillator has been reported in U.S. Pat. No. 5,177,126 (the "'126 patent"). In the '126 patent, two crystals are disposed in the optical path between a mirror and an output coupler. The two crystals operate independently and produce two wavelengths simultaneously. Both crystals are simultaneously active, performing the same function.
There is a need to provide, a frequency conversion device that tunes over a wide range of frequencies without substituting and exchanging one crystal cut at a certain phase matching angle for another crystal cut at a different angle. Additionally, it would be desirable to provide a frequency conversion that uses two crystals to simplify wide tuning, with only one crystal being active at any one time, and the other crystal serving as a beam displacement compensator.